The present invention relates to a disk drive having a servo system for performing head positioning control, and more particularly to a servo system that has a learning function for achieving servo compensation.
Hard disk drives (HDSs) have a servo system for achieving head positioning control. The servo system performs the head positioning control in accordance with the servo data prerecorded on a disk, thereby to move the head to a target position on the disk (a target cylinder or a target track). The servo system is a feedback control system comprising a component of which is a microprocessor (CPU). The CPU serves as the main controller of the HDD.
In manufacturing the HDD, a step known as “servo writing” is performed to record the servo data on the disk. More specifically, a servo track writer (STW) records servo data on the disk in the so-called sector servo scheme after the drive mechanism such as the head-disk assembly is incorporated into the housing of the HDD.
Generally, a number of cylinders (or track), which are concentric, are formed on the disk. Servo data items are recorded in each cylinder, spaced apart in the circumferential direction of the disk. It is desired that the servo data items be recorded at such positions as to define a perfectly circular cylinder as is indicated by the broken circle 31 shown in FIG. 3.
In the course of the servo writing, the disk 1 may undergo a runout, such as decentering or warping, when an impact is externally applied to the HDD or when the clamp mechanism holding the disk has but inadequate dimensional precision. Hereinafter, this undesirable phenomenon shall be referred to as “disk runout.” When a disk runout occurs during the servo writing, the servo data items are recorded along a distorted circle as is indicated by the solid curve 30 in FIG. 3.
In the HDD, the servo system drives and controls the actuator 4, moving the head 2 to the target cylinder (or track) in order to write data in, or read data from, the target cylinder. The servo system controls the actuator 4 in accordance with the servo data the head 2 reads from the disk 1. If the disk 1 much warps and servo data items are recorded along a distorted circuit, the head 2 cannot read the servo data correctly, and the servo system cannot control the actuator 4 as is desired.
A new servo system has been developed for use in HDDs. The new servo system has a learning function. The servo system calculates the amount of a disk runout from the head-positioning error and perform servo compensation on the basis of the amount of disk runout. (The disk runout takes place in synchronism with the rotation of the disk.) The head-position error is calculated from the servo data the head has read, by means of position-error computation. The error represents the distance between the actual position of the head and the target position (in most cases, the center line of the target cylinder).
The new servo system obtains a servo compensation value (a correction value) for reducing the head position error resulting from the disk runout, from the disk-runout value calculated for the head by virtue of the learning function (hereinafter referred to as “learned value”). The servo compensation value thus obtained is supplied to the feedback control system incorporated in the servo system and is added to the control value of the feedback control system. More precisely, the learned value is calculated during each rotation of the disk and stored in a memory. The learned value is read from the memory, and a servo compensation value is obtained from the learned value, whenever necessary to perform the head positioning control. This servo system can control the disk runout, thereby to improve the precision of head positioning. This is because the feedback control system calculates a control value from the head-position error and adds the a servo compensation value to the control value, and performs servo compensation in accordance with the sum of the control value and the servo compensation value. A plant is actually controlled by the servo system is the voice coil motor (VCM) that drives the actuator holding the head.
Having the learning function, the servo system can suppress the instability of head positioning that occurs due to the disk runout. The servo system can therefore help to enhance the precision of head positioning. However, the servo system having the learning function is disadvantageous in the following respects.
First, the servo system may store a low-accuracy learned value (i.e., an erroneous learned value) into the memory. That is, the servo system performs the learning operation when the HDD incorporating the system is started, thus calculating a learned value and stores the value into the memory. If an external disturbance (e.g., an impact) is applied to the disk drive during the learning operation, the servo system fails to calculate an accurate learned value.
Second, the servo system will impair the precision of head positioning if it performs servo compensation in accordance with the sum of the control value calculated by feedback control and the servo compensation value obtained from a low-accuracy learned value read from the memory. In the worst case, the head positioning control the servo system performs may be less precise than the control performed by a servo system having no learning function.